Low-emissivity (low-E) coatings are microscopically thin metal layers that are deposited on a window surface to inhibit heat transfer through infrared radiation across the glass. To improve thermal efficiency of heating or cooling, thin film coatings are applied to the raw glass to affect its reflective and transmissive properties. These coatings result in more thermal efficiency because radiant heat originating from indoors in winter is reflected from the coated windows back inside, while infrared heat radiation from the sun during summer is reflected away by the coated windows, keeping it cooler inside.
High tech glass products such as low-E glazings often have multilayered coatings which may include up to ten metal and oxide layers for anti-scratch, anti-reflective, etc. functionalities. The thermal control is typically provided by a silver layer which is an almost perfect mirror in the infrared range but sufficiently thin to be transparent in the visible range. The layer thus serves as a “heat mirror” or dichroic filter.
There are two primary methods of forming the thin film coatings: chemical vapor deposition and physical vapor deposition. For example, fluorinated tin oxide can be deposited at high temperatures using pyrolytic chemical vapor deposition. In another approach to forming thin film coatings, thin silver layer(s) with antireflection layers are formed by physical vapor deposition (PVD) in large vacuum chambers, requiring multiple deposition chambers to deposit 5 to 10 or more layers in succession. In addition, silver-based films are environmentally unstable and must be protected from exposure to water and air to maintain their properties over time.
With Ag deposited using PVD, many aspects of the Ag film must be controlled to ensure the best quality. In particular, adhesion of the Ag layer to the seed layer must be good. Agglomeration of Ag during initial deposition must be minimal or non-existent. Oxygen content in the Ag layer during nucleation and growth (deposition) must be low. The Ag must be in the preferred orientation (111). The subsequent post-deposition process steps must be controlled to ensure that the layer is not damaged after formation.
To achieve the lowest possible emissivity for a given thickness, the Ag film quality must be low resistivity, i.e., the Ag film should be continuous and of uniform thickness. However, the formation of a continuous film of silver is thermodynamically metastable. Without adequate adhesion to the substrate, thin films tend to reorganize into regions of thicker material and near-empty regions. Attempts have been made to strengthen the weakly adhesive metal/oxide interface by using a thin ZnO layer to increase the wetting and adhesion of the Ag layer. However, Ag adhesion on oxides is generally very poor. The poor adhesion results in agglomeration of Ag during deposition. The Ag is more likely to nucleate in a Volmer-Weber or Stranski-Krastanov growth mode, forming islands initially or after a thin wetting layer is formed.
The wettability can be further improved by adding a “seed layer,” a metallic layer of, for instance, pure titanium, zirconium, or hafnium in the stack, or metal oxide layers. This “seed layer” is normally deposited by a PVD process or reactive magnetron sputtering process. For a metal oxide seeding layer, typically, a thin ZnO layer is used for Ag deposition. The crystalline hexagonal close packed (HCP; 002) orientation of crystalline ZnO film promotes Ag (111) nucleation. ZnO is also preferred (as opposed to a metal layer), because ZnO is a dielectric layer with less absorption than metal material choices that also have the desired characteristics as a seed layer.
Further optimization of ZnO layer includes inserting a metal-doped ZnO layer between the ZnO seed layer and Ag to provide a surfactant effect or using ZnO related alloys. The metallic layer or other “seed layers” must be thin enough that it does not affect the optical properties (does not increase light absorption). However, these solutions have not been entirely successful. The ZnO or alloys exhibit an interfacial energy of ˜1.44 J/m2, so the optimization of Zn alloys is limited to a narrow interfacial energy window. Moreover, oxygen in the metal oxide could diffuse into the metal reflective layer, which would increase its emissivity property, reducing the emissivity performance. In addition, the available process parameters for optimizing the Ag deposition process are limited.